The increased user bit rate requirements that are imposed on cellular mobile radio communication systems of the fourth generation (4G) call for the use of wide-band carriers. The provision of wide-band carriers is only possible in the high frequency regime of the radio spectrum, resulting in increased propagation attenuation due to increased carrier frequencies, and, consequently, reduced radio link distance. The reduced radio link distance dramatically increases the system deployment costs, because the grid of base stations or base transceiver stations has to be tighter to provide for a seamless radio coverage. Furthermore, to increase the spectral efficiency (measured in bit/s/Hz) of the overall communication system, the frequency reuse factor is reduced, i.e. the distance between cells that use the same carrier frequency decreases, resulting in an increased overall inter-cell interference contribution.
Known solutions to mitigate these problems include the deployment of a Relay Base Station (RBS), which increases the effective coverage area of the Super Base Station (SBS) it is associated with by relaying up- and downlink data between the SBS and a terminal. Data that is bound for the terminal then is first transmitted from the SBS to the RBS (in a first “hop”), and then transmitted from the RBS to the terminal (in a second “hop”).
In state-of-the-art systems, both SBS and RBS use a different air interface, i.e. are equipped with additional transceivers, hard- and software, to operate a transmission link with a carrier frequency that is sufficiently spaced apart from the carrier frequencies that are used by the communication system, thus completely avoiding interference between the communication system and the transmission link between RBS and SBS. Said transmission link then may for instance be an optical link or a directional radio link. However, the costs of the RBS and SBS then may dramatically increase.
As a further approach to reduce the above-mentioned problems, Multiple-Input-Multiple-Output (MIMO) transmission techniques are under investigation. The term MIMO refers to the multiple transmission and reception antennas at the input and the output of the spatial propagation channel, whereas the multiple antennas at both the input and the output may be assigned to one station, i.e. be able to co-operate, or to several stations. MIMO transmission techniques exploit the spatial selectivity of the transmission channel by performing spatial or spatio-temporal equalisation of multi-path and fading phenomena at the input of the spatial channel, at the output of the channel or jointly at both the input and the output of the channel. This equalisation mitigates or removes the effects of small-scale and large-scale fading such as multi-path and shadowing and also allows for Space Division Multiple Access (SDMA), i.e. several data signals may be transmitted with the same carrier frequency, with the same code and at the same time instance, and separation of the data signals at the receiver is still possible. The most prominent representative of MIMO techniques is beamforming at the input (transmission side) and/or output (reception side) of the spatial transmission channel, which is of particular interest if only the antenna elements at one side of the transmission channel may co-operate. This is for instance the case if an SBS is equipped with an adaptive antenna array that is composed of several antenna elements. Similar, maximum ratio combining or optimum combining techniques may then be applied to exploit spatial diversity. MIMO techniques can be applied for both frequency-flat channels, but also for frequency-selective channels, where the spatial equalisation of the channel then is extended to a spatio-temporal equalisation of the channel. Whereas the equalisation of the channel helps to reduce the required transmission powers and thus reduces interference, the SDMA approach allows to increase the number of terminals that can be concurrently served by an SBS or RBS.
The prerequisite for the application of MIMO techniques is knowledge on channel parameters such as for instance the position of terminals to which data is transmitted to or received from, or the spatial channel impulse response of the terminal, or the spatial signature of the terminal, or the direction in azimuth and/or elevation of the terminal with respect to the transmitting/receiving antenna array. In typical radio communication scenarios, these channel parameters are generally frequency- and time-variant and can only be estimated from signals received from said terminals. Whereas in Time Division Duplex (TDD) systems, where uplink and downlink take place on the same carrier frequency, the problem of the frequency-invariance of said parameters does not arise, so that only the time-variance (which is mainly determined by the speed of the terminals) of the parameters has to be kept in mind when estimating said parameters during uplink operation and using the estimated parameters for MIMO techniques during uplink and downlink operation, in Frequency Division Duplex (FDD) systems, where uplink and downlink are transmitted in parallel, but on different carrier frequencies, parameters estimated during the reception of signals on the uplink can not be reused for MIMO techniques on the downlink due to the frequency-dependence of said parameters. Thus the application of MIMO techniques that require knowledge on channel parameters at the input (transmission side) of the channel are hard to apply in the context of FDD systems.
The current 4G simulations and analysis indicate that either TDD with globally fixed Tx/Rx turn around periods (all terminals and base stations are then synchronised in a way that the duration and time instance when terminals are allowed to transmit and the duration and time instance when the base stations are allowed to transmit are fixed across the whole communication system) or FDD should be used as duplex techniques. However, both alternatives set out from the assumption that the relation between the amount of uplink data and the amount of downlink data is the same for all base stations, which is generally not the case.